Our long-term objective is to identify the developmental processes by which the unicellular organization o f the egg of Xenopus laevis is transformed into the multicellular organization of the embryonic body axis of this vertebrate. Thus far we have examined three stage-specific processes for the determinative role of each in dorso-ventral development: 1) cortical rotation of the egg, 2) vegetal induction in the late blastula, of the time and place of gastrulation by marginal zone cells, and 3) the progressive acquisition of anterior dorsal fates by marginal zone cells during gastrulation. Each modulates the time, orientation, or extent of the subsequent stage-specific process. Inhibition of any of these of the series leads to the same syndrome of ultimate developmental defects: truncation of dorsal components of the body axis from the anterior end, to a level progressively related to the extent of failure of the process. Complete failure of any process results in radially ventralized embryos, a baseline of development. Dorsalized embryos are formed by exaggerating these processes. In the requested period of support, we plan to define each process further and begin to study the cell-biological basis and consequence of each. For rotation, we will establish what normally orients its direction, what is its immediate local effect in one quadrant of the vegetal hemisphere, and what is the role of microtubules in its operation. For induction, we will establish whether rotation leads to a regional quantitative increase in inductive activity, whether induction affects a maternal cytoplasmic component used by marginal zone cells in initiating gastrulation, and whether induction depends upon exocytotic (secretory) activity by vegetal cells. And finally for gastrulation itself, we will ask about the progressive acquisition of anterior dorsal fates by marginal zone cells, in so far as concerns the reversibility and period of these changes, and their dependence on cell movement and cell interactions with extracellular materials.